This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce images of acquired harmonic signals.
The use of ultrasonic signals which are harmonically related to transmitted ultrasound signals for ultrasonic diagnostic imaging is described in U.S. Pat. No. 5,833,613 (Averkiou et al.) and U.S. Pat. No. 5,879,303 (Averkiou et al.) The ""613 patent describes a number of techniques for imaging with harmonic contrast agents. Ultrasonic contrast agents are comprised of tiny encapsulated microbubbles which, when struck by a transmitted ultrasound wave, will exhibit nonlinear resonance, including resonance at harmonic frequencies of the transmitted wave frequency. This nonlinear resonance will return an echo signal containing the harmonic frequencies in addition to components at the fundamental (transmit) frequency. While the harmonic components are not as great in intensity as the fundamental components, they are nonetheless of relatively significant intensity and can be readily detected and discriminated to provide segmented contrast signal information.
The ""303 patent describes another form of ultrasonic harmonic imaging known as tissue harmonic imaging. Tissue harmonic imaging relies upon the distortion of a transmitted wave which occurs as the wave passes through the tissue of the body. This distortion gives rise to nonlinear signal components including those at harmonics of the fundamental transmit frequency. The tissue harmonic signal components are of a lesser relative intensity as compared to contrast harmonic signal components, but may nonetheless be readily detected and used to form ultrasonic images. As explained in the ""303 patent, tissue harmonic imaging prevents the occurrence of nearfield and other image artifacts which are common to fundamental signal images.
In both contrast and tissue harmonic imaging, the echo signals received can contain both harmonic signal components and fundamental frequency components. These signal components can vary with the type of imaging procedure being performed and the sources of the echo signals. For example, harmonic contrast echo signal components are usually of a lesser intensity than the fundamental echo components, and tissue harmonic signals are generally of a lesser intensity than harmonic contrast components. Fundamental and harmonic contrast components are generally stronger for echoes returned from shallower depths, whereas tissue harmonic components require the passage of a transmit pulse through tissue before the harmonic components develop. All three types of echo signals are subject to depth dependent intensity attenuation and depth dependent frequency attenuation.
The aforementioned ""303 patent takes advantage of these differing characteristics by teaching how produce images which are a blend of fundamental and harmonic signals at different depths and image areas. It would be desirable for such blended images to be formed adaptively in response to actual signal conditions, so that even greater advantage and hence even better images can be formed from fundamental, harmonic contrast, and tissue harmonic signal information.
In accordance with the principles of the present invention, an apparatus and technique are described for adaptively forming images from linear (fundamental) and nonlinear (harmonic) echo information. The fundamental and harmonic content of an echo signal is detected and used to form a composite signal of fundamental and/or echo information which is a function of the relative quality of the two components of the echo signal. In a preferred embodiment the technique is performed on speckle reduced image information. Images are thus formed from fundamental, harmonic contrast, and tissue harmonic signal information which use the respective signal components to maximal advantage and thus higher quality and more diagnostic images.